The present invention broadly relates to an improved electroslag remelting furnace including a liquid-cooled mold adapted to receive a slag bath and a block of a partially molten metal or metal alloy covered by said slag bath during electroslag remelting. The furnace also comprises at least one melt-off electrode formed by said metal or metal alloy which electrode is arranged to extend into the slag bath from an open end of the mold and substantially centrally with respect thereto.
In its more particular aspects, the furnace includes, especially but not exclusively a bottom plate of the type which has high electric conductivity, for instance formed of copper, and which forms a closed end of the mold so as to support the mold and the block formed by remelting. At least one electrical terminal or connection is provided at the bottom plate and is adapted to be connected to an electric current source by means of a conductor. The terminal is disposed in spaced relationship from a mold axis and it is also typically disposed substantially symmetrically with respect to a plane of symmetry which includes the mold axis. Such furnaces are used for remelting alloys, particularly alloyed steel.
In such a furnace the electrode is immersed into the slag bath and it is molten-off or melted by resistance heating. Thereby, a block is formed continuously which rises from the bottom plate arranged at the closed end of the mold. In such a process steel of particularly high metallurgical quality is obtained because of the specifically high purifying effect the slag has upon the melt slowly dropping therethrough. The then solidifying melt, therefore, may have a particularly high degree of purity so that segregations and contaminations are avoided. Another prerequisite to obtain a particularly homogeneous block is an intended uniform solidification of the metal melt. This intended uniform solidification of the melt not only requires a uniformly cooled mold, but the observance of other additional effects.
For example, if the melting-off electrode is not exactly centered or, respectively, if the melting-off electrodes are not arranged symmetrically with respect to the mold axis, then the electrode(s) may melt off asymmetrically; on the other hand, an asymmetric crystallization of the block may thus result. This effect, however, is not only due to an increased heat supply from the electrode directly to the slag and to the metal melt in this region, but it is due, also, to nonuniform motion within the slag in the mold. Consequently, the slag will become preferably cooled in places, in which the motion is low or zero, so that the slag solidifies to a higher degree and forms a thicker cover or blanket of solidified slag in this region as compared to the remaining wall regions.
From this discussion the importance of slag motion will become evident. As such, the slag and the melt, too, are paramagnetic. However, the slag serves as an electric conductor and as such will be subject to the built-up electromagnetic field, whereby a stirring motion occurs within the slag. This particular component of the stirring motion within the slag will be independent of whether a direct-current or a low or a high-frequency alternating-current is used, respectively. It has been observed, particularly in the case of large block diameters, and, especially when using low-frequency alternating-current, for example, in the range of 3 to 10 Hz, that the motion of the slag in the mold will become asymmetric. It has already been assumed that one reason for this asymmetric motion may be the Coriolis force. Furthermore, attempts have been made to achieve mutual balance of the electromagnetic fields by an arrangement of the electric conductors which is as symmetric as possible.
Due to the skin effect and to shielding by the mold difficulties of the kind as mentioned before will be less when higher frequency alternating-currents are used. Up to now no exact scientific explanation could be found for the aforementioned phenomena since the here occurring effects are superimposed upon one another in many ways. Thus, slag motion does not only occur due to forces generated by the electric current; additionally other effects are present which, for instance, result from the temperature gradient within the slag. Furthermore, the electromagnetic fields may be shielded or affected by ferromagnetic material which is always present in a steel mill. While such shielding from electromagnetic fields may eliminate or reduce spurious effects or interferences, the total energy consumption per unit of remelted metal alloy may become substantially increased.